11,792 research outputs found
Efficient Graph State Construction Under the Barrett and Kok Scheme
Recently Barrett and Kok (BK) proposed an elegant method for entangling
separated matter qubits. They outlined a strategy for using their entangling
operation (EO) to build graph states, the resource for one-way quantum
computing. However by viewing their EO as a graph fusion event, one perceives
that each successful event introduces an ideal redundant graph edge, which
growth strategies should exploit. For example, if each EO succeeds with
probability p=0.4 then a highly connected graph can be formed with an overhead
of only about ten EO attempts per graph edge. The BK scheme then becomes
competitive with the more elaborate entanglement procedures designed to permit
p to approach unity.Comment: 3 pages, 3 figures. Small refinement
Sexing up the international
This thesis takes sexuality as its subject matter and uses a methodology informed by postcolonial studies to explore new possibilities for thinking about the international, its construction, and its contemporary politics. I argue that postcolonial readings of sexuality can impel us to rethink the meanings and politics of international theory and to challenge notions that have come to appear fixed and unchanging. The thesis canvasses how such an intervention might occur – calling especially for a focus on the local and the everyday – and considers both the utility and the limits of the contributions sexuality might make to a rethinking of international theory. My arguments are made with reference to a series of specific examples from contemporary East and Southeast Asia: the nationalistically imbued gendered and sexed figures of the national serviceman and the Singapore Girl in Singapore; the political and social repercussions of the trial of former Malaysian Deputy Prime Minister Anwar Ibrahim on charges of sodomy; newly emerging homosexual identities in Hong Kong; and the connections between sexuality and disease that inform the Thai response to HIV/AIDS. These case studies exemplify some of the ways in which sexuality can work to recast traditional scholarly understandings of the international. They also illuminate a series of aspects that shape the encounter between sexuality and the international, encompassing issues of nationalism, globalization, metaphor, spatiality and knowledge politics. Through my analysis of these issues, I argue for a broadening out of the source materials that inform knowledge about the international and the pursuit of alternative modes of reading processes of international change and exchange. I contend that scholarship of the international needs to pay more attention to instances where the borders separating everyday, national and international spaces break down, and where we might detect new forms of knowledge about the nature, politics and functioning of the international realm
Magnetic field sensing with quantum error detection under the effect of energy relaxation
A solid state spin is an attractive system with which to realize an
ultra-sensitive magnetic field sensor. A spin superposition state will acquire
a phase induced by the target field, and we can estimate the field strength
from this phase. Recent studies have aimed at improving sensitivity through the
use of quantum error correction (QEC) to detect and correct any bit-flip errors
that may occur during the sensing period. Here, we investigate the performance
of a two-qubit sensor employing QEC and under the effect of energy relaxation.
Surprisingly, we find that the standard QEC technique to detect and recover
from an error does not improve the sensitivity compared with the single-qubit
sensors. This is a consequence of the fact that the energy relaxation induces
both a phase-flip and a bit-flip noise where the former noise cannot be
distinguished from the relative phase induced from the target fields. However,
we have found that we can improve the sensitivity if we adopt postselection to
discard the state when error is detected. Even when quantum error detection is
moderately noisy, and allowing for the cost of the postselection technique, we
find that this two-qubit system shows an advantage in sensing over a single
qubit in the same conditions
Constructing Smaller Pauli Twirling Sets for Arbitrary Error Channels
Twirling is a technique widely used for converting arbitrary noise channels
into Pauli channels in error threshold estimations of quantum error correction
codes. It is vitally useful both in real experiments and in classical quantum
simulations. Minimising the size of the twirling gate set increases the
efficiency of simulations and in experiments it might reduce both the number of
runs required and the circuit depth (and hence the error burden). Conventional
twirling uses the full set of Pauli gates as the set of twirling gates. This
article provides a theoretical background for Pauli twirling and a way to
construct a twirling gate set with a number of members comparable to the size
of the Pauli basis of the given error channel, which is usually much smaller
than the full set of Pauli gates. We also show that twirling is equivalent to
stabiliser measurements with discarded measurement results, which enables us to
further reduce the size of the twirling gate set.Comment: Fixed typos, added another example and improve presentation
Quantum Computing with Globally Controlled Exchange-type Interactions
If the interaction between qubits in a quantum computer has a non-diagonal
form (e.g. the Heisenberg interaction), then one must be able to "switch it
off" in order to prevent uncontrolled propagation of states. Therefore, such QC
schemes typically demand local control of the interaction strength between each
pair of neighboring qubits. Here we demonstrate that this degree of control is
not necessary: it suffices to switch the interaction collectively - something
that can in principle be achieved by global fields rather than with local
manipulations. This observation may offer a significant simplification for
various solid state, optical lattice and NMR implementations.Comment: 3 pages inc. 3 figure
Efficient variational quantum simulator incorporating active error minimisation
One of the key applications for quantum computers will be the simulation of
other quantum systems that arise in chemistry, materials science, etc, in order
to accelerate the process of discovery. It is important to ask: Can this be
achieved using near future quantum processors, of modest size and under
imperfect control, or must it await the more distant era of large-scale
fault-tolerant quantum computing? Here we propose a variational method
involving closely integrated classical and quantum coprocessors. We presume
that all operations in the quantum coprocessor are prone to error. The impact
of such errors is minimised by boosting them artificially and then
extrapolating to the zero-error case. In comparison to a more conventional
optimised Trotterisation technique, we find that our protocol is efficient and
appears to be fundamentally more robust against error accumulation.Comment: 13 pages, 5 figures; typos fixed and small update
Hierarchical surface code for network quantum computing with modules of arbitrary size
The network paradigm for quantum computing involves interconnecting many
modules to form a scalable machine. Typically it is assumed that the links
between modules are prone to noise while operations within modules have
significantly higher fidelity. To optimise fault tolerance in such
architectures we introduce a hierarchical generalisation of the surface code: a
small `patch' of the code exists within each module, and constitutes a single
effective qubit of the logic-level surface code. Errors primarily occur in a
two-dimensional subspace, i.e. patch perimeters extruded over time, and the
resulting noise threshold for inter-module links can exceed ~ 10% even in the
absence of purification. Increasing the number of qubits within each module
decreases the number of qubits necessary for encoding a logical qubit. But this
advantage is relatively modest, and broadly speaking a `fine grained' network
of small modules containing only ~ 8 qubits is competitive in total qubit count
versus a `course' network with modules containing many hundreds of qubits.Comment: 12 pages, 11 figure
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